This invention relates to the treatment of contaminated liquid by contact with an adsorbent material. The invention has particular, but not exclusive application in the treatment of liquids to remove organic pollutants. Although the invention has particular use in the anodic oxidation of organic compounds, it can also be used for the cathodic reduction of compounds. It can also be used for disinfection.
Adsorbent materials are commonly used in liquid treatment apparatus. Carbon-based such materials are particularly useful, and are capable of regeneration by the passage of an electric current therethrough. The use of carbon-based adsorbents in the treatment of contaminated water is described in the following papers published by The University of Manchester Institute of Science and Technology (now the University of Manchester) in 2004, incorporated herein by reference:
Electrochemical regeneration of a carbon-based adsorbent loaded with crystal violet dye by N W Brown, E P L Roberts, A A Garforth and R A W Dryfe Electrachemica Acta 49 (2004) 3269-3281
Atrazine removal using adsorption and electrochemical regeneration by N W Brown, E P L Roberts, A Chasiotis, T Cherdron and N Sanghrajka Water Research 39 (2004) 3067-3074
The present invention is directed at apparatus for exploiting the ability of the use of an adsorbent material capable of regeneration in the treatment of contaminated liquid. According to the invention, apparatus for treating liquid by contact with a particulate adsorbent material, comprises a reservoir having an inlet and an outlet for liquid to be treated, with a regeneration chamber within the reservoir. Means are provided for recycling adsorbent material along a path including passage through the regeneration chamber and in a body of liquid in the reservoir. The regeneration chamber is defined between two electrodes for coupling to a source of electrical power. In use, a voltage can be applied between the electrodes, either continuously or intermittently, to pass current through the adsorbent material and regenerate it in the manner described in the papers referred to above. The adsorbent material is typically carbon-based.
The treatment and regeneration process can be continuous or semi-continuous. An individual volume of liquid can be treated as a batch, with the adsorbent material being regenerated as the respective batch is treated, or between batch treatments. Some compounds may also be treated within an undivided cell, provided there is no continuous electrical connection between the cathode and anode through the solid conducting adsorbent material. In a continuous or semi-continuous process the flow rate of the liquid through the apparatus is determined and controlled to ensure a sufficient dwell time in contact with the recycling adsorbent.
Apparatus of the invention can be used with a single regeneration chamber, or with a plurality of regeneration chambers in more substantial equipment. Such a plurality of chambers can be in the form of a bank mounted in a common reservoir, which can accommodate circulation of adsorbent material only from either side of the chambers with the chambers closely aligned along an axis of the reservoir and extending to opposing end walls of the reservoir. In another arrangement the bank can accommodate circulation from the sides and ends of a bank of closely aligned chambers within the reservoir and spaced from its end walls. In yet another arrangement circulation can be from around the periphery of each chamber spaced from adjacent chambers in the reservoir. The use of a common reservoir in this way facilitates the recycling of adsorbent material and the flow of liquid through the equipment in greater quantities. A common inlet and outlet can be used for the liquid to be treated, and a single system can be used to recycle the adsorbent. Although the chambers are arranged in a bank, individual electrodes will normally be associated with each chamber for regeneration of the adsorbent.
In apparatus according to the invention, the adsorbent material can be recycled along a variety of different paths, at least a part of which will coincide with the path of liquid to be treated through the reservoir. In that part, the liquid and adsorbent can pass in either the same or the opposite direction. Normally, contaminated liquid will be delivered at the base of the reservoir, and discharged from an upper location, while the adsorbent material follows at least one continuous path within the reservoir.
Recycling of the adsorbent is most easily accomplished by delivery of air to the base or one or more sections of the path which carry the material upwards in that section or sections. This movement carries the material over a boundary at the top of the regeneration chamber, in which it then falls under gravity. As it moves through the regeneration chamber, the applied voltage causes a current to flow through the material, destroying the adsorbed pollutants. The breakdown products can be released in gaseous form, and treated separately as appropriate.
The use of air to recycle the adsorbent material is of course beneficial in itself to the treatment process. It aerates the contaminated liquid, as well as agitating the adsorbent material as it is recycled, thereby enhancing its exposure to the contaminated liquid. Incoming liquid can also be used to entrain and assist in circulating the adsorbent from the bottom of the regenerating chamber. This can be of benefit when treating liquids containing surface active compounds which could result in foaming. Of course, different fluids can be used to accomplish different treatments of various liquids in the apparatus.
The recycling path for the adsorbent material and the regeneration chamber can be arranged differently in a reservoir, depending on the requirements for liquids to be treated, contact time of liquids to be treated and the amount of material to which the liquid should be exposed. In a preferred arrangement, the regeneration chamber is located centrally within a reservoir, with the adsorbent material being adapted to fall through it, and be recycled upwardly within the reservoir and through the liquid to be treated on the outside of the chamber, A convenient design of apparatus has the regeneration chamber located between two treatment chambers, one on either side thereof, in what is effectively a two-dimensional arrangement. The electrodes for the regeneration chamber can then be disposed on opposite faces thereof, these faces being different from the sides against which the treatment chambers are defined. This arrangement can though, of course be extended to three-dimensions with the regeneration chamber being surrounded by a plurality of treatment chambers. These arrangements can also be reversed, with a single treatment chamber located centrally either within an annular regeneration chamber, or surrounded by an array of regeneration chambers.
Adsorbent materials suitable for use in this invention are electrically conducting solid materials capable of easy separation from the liquid phase. The material may be used in powder, flake or granular form. Whilst the particle size is not critical, the optimum size will depend on the adsorbent properties. The material used and particularly the particle size is a compromise between surface area, electrical conductivity and ease of separation. Preferred materials are graphite intercalation compounds (GICs). A particularly preferred GIC is a bi-sulphate intercalated product.
It can be formed by chemically or electrochemically treating graphite flakes in oxidising conditions in the presence of sulphuric acid. However a large number of different GIC materials have been manufactured and different materials will have different adsorptive properties which will be a factor in selecting a particular material.
Reducing the particle size of the adsorbent material will significantly increase the surface area available for adsorption. However reducing the particle size will make separation of the solid phase more difficult. in the practice of the invention a typical particle size is 0.25-0.75 mm. Very fine particles (<50 microns) can be used as the adsorbent material as these can be separated from the liquid phase easily if an organic polymer is used as a flocculent. This organic flocculent is then destroyed by regeneration. The use of other materials of lower electrical conductivity and density would benefit from larger particles.
The higher the electrical conductivity of the adsorbent material, the lower will be the voltage required across the cell and so the lower power consumption. Typical individual GIC particles will have electrical conductivities in excess of 10,000 Ω−1cm−1. However in a bed of particles this will be significantly lower as there will be resistance at the particle/particle boundary. Hence it is desirable to use as large a particle as possible to keep the resistance as low as possible. Hence a bed of fine wet particles has been shown to have an electrical conductivity of 0.16 Ω−1cm−1 compared with 0.32 Ω−1cm−1 for a bed of larger particles. As a comparison a bed of granular and powdered activated carbon would typically have electrical conductivities of 0.025 and 0.012 Ω−1cm−1 respectively.
The preferred GIC used in the practice of the invention is in flake form, and typically has a composition of at least 95% carbon, and a density of around 2.225 g cm−3. However flake carbons can be used as the starting materials for producing GICs with significantly lower carbon contents (80% or less). These compounds can also be used within the cell, but are likely to result in slightly higher voltages across the electrochemical regeneration stage. Other elements will also be present within the GIC, these compounds are dependent on the initial composition of the flake graphite and the chemicals used to convert the flakes into intercalated form. Different sources of graphite can produce GICs with different adsorptive properties.